OK so it's a relatively brittle plastic then, but I guess that you've found that out already
What I would suggest is that you try is going up in drill sizes until you are at the sime size as the small diameter of the thread in the hole, but when you are close to size, do it by hand rather that with a power drill, as you do not want to damage the thread. (these bolts take quite a load when your car is at speed). You then need a tap that matches the size of the bolt (any reasonable tool shop should be able to help you with this) and a wrench to turn it.
The tap will literally re-cut the thread, so make sure that is in the original thread straight and start gently with just you fingers. Be careful tho as taps are a very hard grade of steel and if you break it in the hole you will not be able to remove it.
A little WD40 applied to the tap will make it a bit easier
Last edited by Pat.w; 10th December 2008 at 07:45 AM.